A magnetometer is a device which measures magnetic field and provides a signal which is related to the field intensity. Magnetometers have long been known and have many applications. There are several types of magnetometers, including those which are mechanical in nature and those which are electrical and electronic. One popular type of electronic magnetometer is called a flux-gate magnetometer. Flux-gate magnetometers operate by saturating and unsaturating a magnetic core about which a sense winding is provided. The operation of saturating and unsaturating the core is achieved with a drive signal applied to a drive winding on the same core. The drive winding alternately concentrates flux lines through the sense winding and allows them to relax. This arrangement thus serves as a gate or valve for the magnetic flux. An external magnetic field interacts with the magnetic field generated by the sense winding in a way which can be detected through electronic circuitry connected to the sense winding. The output signal of a flux-gate magnetometer is a voltage which is linearly proportional to the external magnetic field. The reader will appreciate that compasses can readily be constructed from devices having these properties. Such compasses are useful, for example, in automotive, marine, and aircraft navigation.
In compass applications, two or more magnetometers are generally employed, at least one per axis. Electronic circuitry processes the information about field changes along the two orthogonal axes to derive heading information. We may consider each axis separately as the two magnetometers are normally of the same design and construction. More than two axes are sometimes used, to increase the signal-to-noise ratio of the measurement, or to detect magnetic dip angle.
Prior magnetometers comprise two or more coil windings and a number of electronic circuits. One of the windings is driven by an electronic signal and at least one of the other windings then produces an output which depends not only on the driving signal, but also on changes in flux within the driven and sensing windings produced by movement or reorientation of those windings. For example, in U.S. Pat. No. 4,305,035, issued Dec. 8, 1981, in the name of Dedina O. Mach et al., two coils are used for sensing along each axis. Each coil pair has its own electronics and each coil pair is wound on separate, small strips of core material; the coils are positioned in a gimballed fixture for use as an electronic compass. One of the coils in each pair is a driving coil to which a driving signal is applied and the other coil is a field-sensing coil. In a two-axis compass application, each of the sensing coils senses only the component of magnetic field in one plane, with one output being representative of the field along the axis of one of the coils and the other being representative of the field along the axis of the other coil. The driving signal applied to each driving coil is of triangular waveshape, to saturate the coil core in alternating, opposite directions. An applied field has an additive effect during one-half of the excitation cycle and a subtractive effect during the other one-half cycle, producing a tendency to establish a net dc level in the sensed waveform. This dc component is removed and the signal required to do so is used as a representation of the applied field strength. As seen in FIG. 1 of that patent, the there-indicated embodiment of the invention not only requires the aforementioned multiple coils, but also (for each axis) six operational amplifiers and numerous additional components.
A flux-gate magnetometer which employs three coils and one drive winding for two-axis sensing is marketed by KVH Industries, Inc., Middleton, R.I., as part of that company's KVH PC103 flux-gate compass and related products. According to KVH literature, its sensor uses a toroidal flux-gate magnetometer with a free floating ringcore in the center. The core is made of a stainless steel bobbin wound with Permalloy (a registered trademark) brand tape. A drive field is applied to the core and the external field interaction with the drive field produces an asymmetric change of core flux. This change in core flux is detected by a secondary winding over the core and the resultant signal is then processed.
Other multiple winding flux-gate magnetometer compasses are shown, for example, in U.S. Pat. No. 3,899,834 and in U.S. Pat. No. 4,277,751.